We propose to study the mechanisms underlying long-term habituation of the gill and siphon withdrawal reflex of Aplysia in order to examine the relationship between the mechanisms underlying short- term memory which lasts minutes to hours and long-term memory which lasts days or weeks. In short-term habituation, a critical locus of cellular change is at the synaptic connections between the sensory neurons and the motor neurons. The same synaptic connections are depressed during long-term habituation. This finding opens up the possibility for exploring directly the mechanisms of long-term memory of habituation and their relation to short-term memory. While the intact animal preparation will still be the system of choice to characterize the properties of long-term habituation, we will take advantage of two reduced preparations to analyze its mechanisms in more detail. The first one is an isolated reflex preparation of the gill and siphon withdrawal reflex. The second one is a dissociated cell culture system in which the monosynaptic component of the reflex is reconstituted. Both preparations will allow us to use a variety of pharmacological tools to distinguish short-term habituation from long-term habituation. The simplicity of the cell culture system will permit us to focus on a critical synapse of the reflex to study the electrophysiological, biochemical and anatomical changes that are associated with long- term synaptic depression. Finally, we will take advantage of the fact that the gill and siphon withdrawal reflex can also undergo sensitization to compare the mechanisms of long-term habituation to the mechanisms of long- term sensitization.